The present invention relates to a marine seismic cable system and more particularly relates to a means for (1) protecting the depth control devices which are used to maintain a seismic cable at a desired depth during a seismic survey operation, and (2) protecting buoy or anchor lines in an area being surveyed from being damaged by the depth control devices.
In a typical marine seismic operation, an electronically equipped vessel tows both a seismic source and a seismic cable or streamer through an area to be explored. The source is actuated to generate signals which in turn reflect off various strata underlying the marine bottom. These reflected signals are received by each of a plurality of geophones or the like which are spaced along the length of the cable. The received signals are recorded and processed to produce the desired seismic record. Due to the criticality of all measurements involved, it is important that the cable be towed and maintained at a predetermined, known depth during the operation.
One of the most successful techniques for maintaining the cable at a desired depth during a seismic operation involves the use of depth control devices of the type commonly referred to as hydroplanes, paravanes, or simply as "birds". Such devices are attached at spaced points along the cable and are well known in the art, e.g., see U.S. Pat. Nos. 3,375,800; 3,434,446; 3,774,570; and 3,896,756. A basic depth controller of this type has a torpedo-shaped hull made of plastic material which is made in two parts and hinged together to facilitate assembly directly onto the cable. At least one set of adjustable wing members is pivotably mounted near the forward portion of the hull and is controllable (preferably by remote control electronics carried in the hull) so that the wings may be moved up or down to cause the cable to rise or sink as desired.
Unfortunately, due to its design a depth controller of this type is susceptible to damage when certain obstacles are encountered in a survey area. One such obstacle is the lines or mooring cables which connect a lobster or crab trap to its marker buoy on the surface. If a seismic cable is towed through an area where such traps are set, a buoy line may contact and ride along the seismic cable until it engages a depth controller or "bird" on the cable. The buoy line which is normally a length of high strength nylon cord or the like may lodge either (1) within the slot formed by the mating halves of the hull of the bird, or (2) in the space between the wing of the bird and the hull. In either event, continued towing of the cable causes the buoy line to develop a "sawing" effect which can seriously damage the expensive bird and thereby causes a substantial delay in the seismic operation. The bird can also damage the line which connects the lobster or crab trap to its marker buoy.
As will be more fully discussed below, attempts have been made to avoid part of these problems by providing shield means on the hull just forward of the wings but this remedy has only met with partial success. Since the ability of wings to move must be maintained for a bird to function as intended, the shield means cannot come into contact with the wings and therefore a space still exists between the shield means and a wing in which a buoy line or the like can find its way.